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Re: ok, looks like I got mixed up on Herrerasaurus a few posts back



On Sun, Oct 4, 2009 at 12:22 PM, B tH <soylentgreenistrex@yahoo.com> wrote:
> So, it's not (thankfully) a sauropodomorph - it is safe to say it's at least 
> a saurischian, basal or not? ÂI obviously don't know the anatomy that makes 
> the professionals question exactly where it goes, and mentally I'll consider 
> him a theropod ... I just don't want him stuck somewhere close with 
> herbivores.

Taxa aren't based on diet -- giant pandas are herbivorous carnivorans.
Within our own "family", most species are omnivorous, but there have
been both herbivores (Gorilla, some Homo sapiens individuals) and
carnivores (Homo neanderthalensis).

Taxa are often based on phylogeny. Based on most of the phylogenetic
hypotheses I've seen lately (and I could be missing something),
theropods actually share more recent ancestry with sauropodomorphs
than they do with herrerasaurids. To put it in relative chronological
order:

1. There is an ancestral population of dinosaurs. (These would likely
have looked more or less similar to, e.g., Eoraptor.)
2. This population splits into saurischian and ornithischian branches.
3. Saurischians diversify into many populations.
4. One of the saurischian populations, called the ancestral
eusaurischian population, splits into sauropodomorph and theropod
branches.
5. Another population of saurischians gives rise to Herrerasaurus
ischigualastensis.

If you wish to familiarize yourself with the phylogenetic definitions
of names such as "Saurischia", "Ornithischia", "Dinosauria",
"Eusaurischia", "Sauropodomorpha", and "Theropoda", I highly recommend
this website: http://taxonsearch.org/
(Note, however, that the definitions Sereno endorses are not
necessarily used by everyone. But the site lists all published
definitions, so you see all of the differing opinions.)

In short, Herrerasaurus is probably not a theropod, but probably is a
saurischian. Remember, "theropod" does not mean "sharp-toothed,
bipedal meat-eater". There are herbivorous theropods (e.g.,
therizinosaurids, some anseriforms, etc.) and toothless theropods
(oviraptorids, avians, etc.). And there are sharp-toothed, bipedal
meat-eaters which are not theropods (postosuchids, herrerasaurids,
Lagosuchus, etc.). Try to avoid typological thinking, and think
instead in terms of phylogeny.
-- 
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies
Glendale, California
http://tmkeesey.net/